First, I'm nowhere near the most qualified to answer, but as I'm the first to respond, I'll try my best.

Let's just look at the cease fire proposal between government aligned fighters and the moderate rebels. Assad has no intention of leaving, he's currently secure in Damascus. Hezbollah and Iran will continue to supply funds, equipment, and manpower to him; the SAA and NDF by now are saturated with fighters who truly want him in power. As such, these forces have no reason to stop fighting.

For this reason alone the moderate opposition cannot trust a cease fire to hold. So they won't enter one, even if they wanted one. They're fighting to overthrow Assad. A cease fire keeps him in power. Anything less than a victory is a loss.

Given these two camps cannot coexist, it is impossible for them to join as allies against Nusra and ISIS.

(Also, the analysis requires a quick reminder that big portions of the 'moderates' AKA not ISIS?Nursa/Ahar al-Sham in the north include the old Levant Front, the existing Islamic Front, and other umbrella groups are Islamists. They more closely identify with Nusra than a 'secular Crusader ally Assad'. So why would they turn on Nusra and join a 'secular Crusader Assad', and 'polytheistic Western puppet group' alliance?)

Now the international coalition which would include Assad, moderate rebels, Kurds, and Gulf/Western nations? Impossible. The West won't allow itself to support a dictator that uses chemical weapons, targets civilians, etc. (It's no longer the 70s, 80s). Also, the West does not want to inflame Arab/Kurdish tensions more by having them press into more Arab areas. Plus, Turkey would lose its' mind if Kurdish forces stretched all across their border. Furthermore, the West and its allies have been targeted many times by Hezbollah. They don't want to help Hezbollah, even in a non zero sum capacity.

That's just a quick summary of the boldest flaws in the plan.